The present invention relates to the data processing field, and more particularly, to an ordering mechanism, ordering method and computer program product for implementing peripheral component interconnect local bus (PCI) peer to peer functions.
PCI local bus (PCI) ordering rules require that write operations must pass read operations, but that read operations cannot pass write operations. Many interconnect networks with strongly ordered but independent command classes do not implicitly support such ordering rules. Intelligent input/output (I2O) architecture is an emerging industry standard which supports connecting intelligent IO devices, which allow peer to peer transfers.
Due to I2O/PCI ordering requirements in support of peer to peer transfers, dead lock conditions in interconnect networks have been discovered. The deadlock conditions were caused by the fact that some interconnect networks are strongly ordered for each command class, and use a busy based flow control where the destination sends a busy back to the source for a command if the destination cannot accept the command. There is no ordering requirements between command classes. Once a command is busied by the destination, the interconnect network guarantees that the busied command will be the next command in that command class, sent to the destination. In some interconnect networks more classes have been added with interdependencies between the classes. By adding additional classes and having outstanding commands in one class block the issuing of commands in another class until the first commands have been executed, the ordering restrictions can be accommodated and dead lock conditions can be prevented. This results in additional complexity and performance degradation.
An alternative protocol implements a credit based system where the source always knows how many commands the destination can receive. This allows the interrogation of a command to decide to send the command or retry the command back to the source, and solve the deadlock condition.
An improved mechanism is needed for implementing PCI local bus (PCI) peer to peer functions.
Principal objects of the present invention are to provide an ordering mechanism, ordering method and computer program product for implementing PCI local bus (PCI) peer to peer functions. Other important objects of the present invention are to provide such an ordering mechanism, ordering method and computer program product for implementing PCI local bus (PCI) peer to peer functions substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.
In brief, an ordering mechanism, ordering method and computer program product for implementing PCI local bus (PCI) peer to peer functions. When a read command is received, checking for available resource is performed. Responsive to not identifying available resource, a retry read command is sent.
In accordance with features of the invention, responsive to sending the read command, checking for the retry read received command is provided. Responsive to identifying the retry read received command, the read command is resent.